1. Field of the Invention
The present invention relates to a piston having a skirt for an internal combustion engine, and particularly, to the shape of a piston for reducing the piston slap sound of a piston.
2. Description of the Related Art
In a piston for an internal combustion engine, which includes a piston pin having an axis offset with respect to an axis of a cylinder, and a skirt formed into an intermediately bulged barrel type, a technique is known from Japanese Utility Model Publication No.53-40564, of partially cutting away that upper portion of the skirt located on a side opposite to the offset side of the piston pin, which is liable to cause a scoring and a spotting between such portion and a wall surface of a cylinder bore, thereby preventing the occurrence of a seizure.
The skirt of the barrel type is intended to reduce the piston slap sound generated by the collision of the piston against the wall surface of the cylinder bore, but in the above known piston, an improvement has been made in the barrel shape of the upper portion of the skirt from the viewpoint of the lubrication of the piston. This does not contribute to a reduction in piston slap sound.
Accordingly, it is an object of the present invention to ensure that reductions in piston slap sound in both of an expansion stroke and a compression stroke are reconciled effectively in consideration of the differences in behavior of the piston and in thrust force between the expansion stroke and the compression stroke.
To achieve the above object, according to a first aspect and feature of the present invention, there is provided a piston for an internal combustion engine, including a barrel-type skirt comprised of an upper skirt portion provided above a maximum-diameter portion and a lower skirt portion provided below the maximum-diameter portion, the upper and lower skirt portions having radii smaller than that of the maximum-diameter portion, wherein a decrement in radius of the lower skirt portion on an anti-thrust side is set smaller than a decrement in radius of the lower skirt portion on a thrust side.
With the above arrangement, when the piston is swung in an expansion stroke, while the lower skirt portion, the maximum-diameter portion and the upper skirt portion of the piston on the thrust side are put sequentially in the named order into abutment against a wall surface of the cylinder bore, the swinging movement of the piston can be permitted, and the collision of the skirt against the wall surface of the cylinder bore can be moderated to reduce the piston slap sound, owing to a sufficient decrement in radius of the lower skirt portion ensured on the thrust side.
In a compression stroke, the lower skirt portion on the anti-thrust side is first put into abutment against the wall surface of the cylinder bore, and on the subsequent swinging movement of the piston, the upper skirt portion on the anti-thrust side collides against the wall surface of the cylinder bore to generate the piston slap sound. However, since the decrement in radius of the lower skirt portion on the anti-thrust side is set small, the inclination angle of the piston that is generated upon the abutment of the lower skirt portion on the anti-thrust side against the wall surface of the cylinder bore can be suppressed to a small value, and the shock provided upon the collision of the upper skirt portion on the anti-thrust side caused by the subsequent swinging movement of the piston in an opposite direction can be alleviated to reduce the piston slap sound.
The decrement in radius of the lower skirt portion is provided to be asymmetric on the thrust side and the anti-thrust side in the above manner, whereby the reduction in piston slap sound can be effectively achieved in both of the expansion stroke and the compression stroke.
According to a second aspect and feature of the present invention, there is provided a piston for an internal combustion engine having a skirt, wherein the curvature of a cross section of the skirt on an anti-thrust side is set smaller than that of the cross section of the skirt on a thrust side, and at least a portion of the cross section of the skirt on the anti-thrust side is formed in an arc of circle.
With the above arrangement, the curvature of the cross section of the skirt on the anti-thrust side is set smaller than that on the thrust side, and at least a portion of the cross section of the skirt on the anti-thrust side is formed in the arc of circle. Therefore, the surface pressure of contact of the piston with the wall surface of a cylinder bore can be reduced in a compression stroke to reduce the piston slap sound and to enhance the seizure-resistance performance.
According to a third aspect and feature of the present invention, in addition to the second feature, there is provided a piston for an internal combustion engine, wherein the arc of circle is formed with a center angle of 10xc2x0 or more on each of opposite sides of an axis extending through a central axis of the piston to the thrust side and to the anti-thrust side.
With the above arrangement, the arc of circle is formed with the center angle of 10xc2x0 or more on each of the opposite sides of the axis. Therefore, the surface pressure of contact of the piston with the wall surface of the cylinder bore can be reduced sufficiently in a compression stroke.
The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the accompanying drawings.